When I was sixteen I remember thinking, if there is no God it does not matter what I do. So I shot some birds with an air rifle. Then one of them was wounded but not killed and it hid from me under a bush. I saw it looking at me and I realized it did matter. I had cruelly and pointlessly injured a harmless little bird and I felt bad.
It taught me that even if there is no God and no punishment for anything we do we should still have compassion.
Trying to stick to the subject, not everyone has compassion. There are many many people that are mentally ill, selfish to the point of not caring about others etc. Ultimately, we need God because we are incomplete without him, but, for people that don't "naturally" have compassion and love for others, they need God to change them from the inside out.
Not everyone is able to 'self police' - but like I said, we need God not to keep us in line, but to give us REAL life.